Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. are powered by a source of relatively low-voltage DC power. Because power is generally delivered through a wall outlet as high-voltage AC power, a device, typically referred to as a power supply, is required to transform the high-voltage AC power to low-voltage DC power. The low-voltage DC power may be provided by the power supply directly to the device or it may be used to charge a rechargeable battery that, in turn, provides energy to the device, but which requires charging once stored energy is drained. Typically, the battery is charged with a battery charger that includes a power supply that meets constant current and constant voltage requirements required by the battery. In operation, a power supply may use a controller to regulate output power delivered to an electrical device, such as a battery, that may be generally referred to as a load. More specifically, the controller may be coupled to receive feedback information about the output of the power supply in order to regulate power delivered to the load. The controller regulates power to the load by controlling a power switch to turn on and off in response to the feedback information to transfer energy pulses to the output from a source of input power such as a power line.
One particular type of power supply that may be used is a flyback power supply. In one type of flyback power supply, an energy transfer element isolates the input of the power supply from the output of the power supply. The energy transfer element provides the isolation that prevents DC current from flowing between the input and the output.
A typical way to generate a feedback signal for use by the controller in a flyback power supply and still maintain isolation is to use an optocoupler on the output to send a signal to the controller. Another known way is by using sensing circuitry on the input side of a galvanically isolated power supply, also referred to as “primary-side control”. One example of primary side control is to include an additional “auxiliary” or “bias” winding in the energy transfer element that is magnetically coupled to the output side to generate a voltage that is representative of the output of the power supply. Additionally, the bias winding may provide a voltage that is representative of an input voltage of the power supply.
However, utilizing the waveforms generated on the auxiliary winding often requires the inclusion of numerous electrical components external to the controller to condition the signals for receipt by the controller.